1. Field of the Invention
The present invention relates to a device for assay of a liquid sample, and more particularly, to a dry type device for colorimetric assay of at least one component in a liquid sample, for example, body fluids such as blood.
2. Description of the Related Art
Assay of body fluids such as blood has been done in hospitals or by assay experts. Recently, with the development of a dry type assay device, a patient can carry out the assay easily, quickly and precisely. For example, the patient samples a small amount of his or her blood and determines the glucose content in the blood so as to control the sugar content in his or her food.
Typical examples of the dry type assay devices are disclosed in U.S. Pat. Nos. 3,298,789 and 3,630,957 and Japanese Patent Publication No. 33800/1974. With the device of the U.S. Patent, the sampled whole blood is placed on a reagent pad covered with an ethylcellulose layer. After a predetermined period of time, the blood sample is washed out with water, and the developed color of the reagent pad is detected. If erythrocytes remain on or in the reagent pad, they interfere with the detection of the developed color. To avoid such interference, the cellulose layer is provided to prevent penetration of the erythrocytes into the reagent pad and to facilitate removal of the erythrocytes. With the device of the above Japanese Patent Publication, a reagent pad comprises a water-resistant film which prevents penetration of the erythrocytes into the reagent pad.
Various special equipment has been developed and used in order to simplify the control of time for the assay and determination of the degree of developed color by the patient and also to increase the reliability of data. With such equipment, the patient punctures a blood vessel and then he or she should start a time control means of the equipment at the same time as he or she applies a drop of blood on a reagent layer of the assay device such as an assay stick.
However, it requires some skill to start the time control means and to apply the drop of blood on the reagent layer simultaneously. In most cases, just after the application of the drop of blood on the reagent layer, the patient starts the time control means. Therefore, the reliability of the data is not satisfactory. Further, it is troublesome for the patient to apply the drop of blood and to start the time control means simultaneously. In addition, the patient should wipe out the blood after a predetermined period of time from the application of blood and then set the assay device in the measuring equipment. Such procedures are troublesome for the patient and deteriorate the reliability of the data.
Among the above problems, troublesome wiping of the blood can be neglected by the assay devices disclosed in U S. Pat. No. 3,992,158, DE-A-3 029 301, Japanese Patent Publication Nos. 21677/1978 and 164356/1980 and Japanese Patent Kokai Publication No. 24576/1981. For example, the device disclosed in U.S. Pat. No. 3,992,158 is a multi-layer integral assay element comprising a transparent support, a reagent layer and a porous spreading layer which are laminated successively. In use, when the sample is applied on a spot of the spreading layer, it spreads in a transverse direction and migrates into the reagent layer to develop the color in the reagent layer. Then, the degree of color development is observed through the transparent support. With such device, the color development is measured without the removal of the applied blood. However, such device cannot overcome the problem that the control of time should be started simultaneously with the application of blood.
To avoid the troublesome time control, EP-A-256 806 and Japanese Patent Kokai Publication No. 101757/1988 disclose an equipment which can measure the blood color and the developed color with two beams of light having different wavelengths, one of which is absorbed by the erythrocytes and the other of which by the developed color from an opposite side to the sample applied side. In measurement, a drop of blood is applied on the reagent layer of the dry type assay device as shown in FIG. 1 which is set in the measuring equipment. That is, when the drop of blood is applied on the reagent layer 2 supported by a support 1 of the dry type assay device, reflectance of the light having the wavelength which is absorbed by the erythrocytes decreases, and the start of such decrease of reflectance is automatically detected by the equipment and regarded as the application of blood on the dry type assay device.
However, this type of equipment has the drawback that it tends to be influenced by stray light. That is, in this equipment, the application of sample and the degree of color development are both measured as changes of amounts of reflected lights at the same point indicated by the arrow C in FIG. 1. The timing of the sample application is detected as the change of amount of reflected light when the sample is applied. In general, the reflectance of light is expressed in terms of a percentage of the amount of light reflected on the reagent layer to the amount of light reflected on a standard white plate of magnesium oxide. With this equipment, since the amount of light from the reagent should be measured on the opposite side to the side which is illuminated with the measuring light, the reagent layer is transparent to some extent, and therefore, the amount of light received by light receiving means of the equipment is a sum of the light reflected by the reagent section and light from ambient light sources (e.g. the sun or room lamps) which passes through the reagent section and reaches the light receiving means. In measurement with this equipment, particularly when the room is light, as a finger reaches the reagent layer to apply the blood, the surface of reagent layer is shadowed with the finger. Then, even if the blood is not actually applied on the reagent layer, the reflectance is changed so that the timing of the start of measurement may not be correctly detected. In addition, this equipment may have an insufficient dynamic range, since the application of the sample and the degree of color development are both measured by the measurement of reflectance on the reagent layer, so that the degree of color development is measured by using the reflectance of the reagent layer including the blood as a background. Since the blood absorbs not only light at a specific wavelength but also light from the developed color and further light outside the visible light range, when the reflected light is measured against the erythrocyte component as the background, the decrease of optical dynamic range cannot be avoided. The decrease of optical dynamic range decreases the accuracy of measurement. The transparency of the reagent layer which is necessary for the detection of application of the sample is one of the causes for the decrease of dynamic range.